The present invention relates to a developing apparatus to develop an electrostatic latent image on an image carrier and to form a visible image, and relates to an image forming apparatus such as a copier, printer, facsimile device, or similar devices, which is provided with the developing apparatus for image formation.
Conventionally, the following image forming apparatus is widely known: developer is carried on the surface of a developer carrier; the developer is conveyed to a developing area opposed to an image carrier carrying thereon an electrostatic latent image; and while AC voltage is continuously superimposed on DC voltage and these voltage are applied onto the developer carrier, the electrostatic latent image is developed and a visible image is formed.
Further, when the electrostatic latent image is developed, contact development such as magnetic brush development or the like, by which developer layer carried on the developer carrier is slid-contact with the image carrier; and non-contact development by which, in the developing area, toner particles are flied in the air layer from the developer carrier toward the image carrier, and thereby the electrostatic latent image is developed, are well known.
FIG. 10 is an enlarged typical view showing toner movement in the developing area in which the image carrier is opposed to the developer carrier.
For example, when non-contact development is conducted in the developing area, at an edge portion at which a gap exists in latent image potential on the image carrier, toner t in the vicinity of the edge portion on the opposing developer carrier (developing sleeve) is absorbed in a high density portion (a large potential area), and a toner adhering amount near the border portion increases more than expectation, and thereby, sometimes image density is increased. Further, sometimes an area onto which the toner t adheres more, spreads widely (hereinafter, such a phenomenon is referred to as "absorption").
Reversely, there is a case where the toner t adhered onto a low density portion (small potential area) adjoining a high density portion is absorbed in the high density portion as shown by a dashed line in FIG. 10, and as the result, the toner t hardly adheres onto the low density portion and the toner t on the low density portion is lost whitely (hereinafter, such the phenomenon is referred to as "white loss").
These absorption and white loss problems frequently occur in a non-contact development method.
A mechanism of generation of these absorption and white loss will be detailed more using a typical view in FIG. 10.
In the air layer in which toner is separated from the developer carrier (developing sleeve) and adheres onto the image carrier (photoreceptor), electric lines of force (shown by arrows in the drawing) are generated from an area (space) above the low density portion to an area (space) above the high density portion. These electric lines of force perpendicularly act on equipotential lines in the drawing.
Normally, it is considered that an adhering amount of toner onto an electrostatic latent image on the image carrier is determined unconditionally corresponding to latent image potential. However, actually, it is considered that toner t on the developer carrier opposed to the low density portion flies along the electric line of force, therefore an adhered amount of toner on the high density portion side at an edge portion is larger than that expected from the latent image potential, and an adhered amount of toner on the low density portion side at the edge portion is smaller than that expected from the latent image potential.
That is, when an AC continuous wave is applied onto the developer carrier, because the direction of electric field is cycled for each half cycle due to AC voltage, the movement direction of toner t is also changed for that time. Accordingly, toner flying speed in the vicinity of the changing point of the movement direction of toner t is decreased, therefore toner t easily moves along the electric line of force shown in FIG. 10, and the adhered amount of toner on the high density portion side at the edge portion is increased, resulting in generation of absorption. Due to this absorption, the adhered amount of toner on the low density portion side at the edge portion is decreased, resulting in white loss.
Further, even if toner t flies to the low density portion on the image carrier, the toner t finally lands on the high density portion when the toner t repeats bounding on the image carrier. This is also considered to be a cause for absorption and white loss.
In the non-contact development, in order to keep the developer layer on the developer carrier and the image carrier surface non-contact, it is necessary that the distance between the developer carrier surface and the image carrier surface is set to be longer than the case of the contact development. Due to this distance setting, curvature of the electric line of force becomes large, and absorption and white loss become conspicuous.
Further, although these absorption and white loss phenomena are not so conspicuous as in the case of the non-contact development, these phenomena are also generated in the contact development.
Concerning these absorption and white loss, recently it is discovered that, in the voltage waveform with which bias voltage in which DC voltage is superimposed onto AC voltage, is applied, when voltage having a blank portion to intermittently stop AC components, and having a waveform with which only DC components (blank pulse waveform) is impressed, is applied onto the developer carrier in the blank portion, the problems can be solved.
Well known references of the developing method using a blank pulse waveform in the conventional developing bias voltage are shown as follows: Japanese Tokkaihei No. 7-311497, No. 8-160725, No. 5-35063, Tokkaisho No. 60-134262, No. 60-53968, Tokkaihei No. 7-295373, No. 6-348117, No. 7-92786, and the like.
However, while the developing method shown in these well known references are effective for absorption and white loss, AC voltage which triggers the toner to be separated from the developer carrier surface, is not impressed on the developer carrier in the blank portion in which impression of AC voltage is stopped. Accordingly, a separated amount of toner is decreased, and there is a problem that the developability (toner adhesiveness) is decreased in general as compared to the continuous waveform portion which is not a blank portion.